52 C. R. VAN HISE 



ing that the igneous material is uniformly distributed vertically 

 through the outer five miles, the material would occupy a surface 

 space of about 4,800,000 square miles, with consequent surficial 

 contraction and thickening of the remaining material of the 

 crust. The surficial shortening of the original crust involved 

 would in this case be about one-half as great as that due to 

 secular cooling throughout geological time, as calculated by 

 Mallet, and more than one and one-half times as great as that 

 calculated by Dutton (see p. 41), even if it were supposed that 

 the entire contraction were available for crustal corrugation. 



Of course the above figures are hypothetical. The purpose 

 of introducing them is to show the relative importance of crustal 

 corrugation as a result of intrusion and nucleal Contraction due 

 to the transfer of magma, and to emphasize the fact that vul- 

 canism is probably one of the great causes for shell corrugations, 

 for two reasons. The intrusives occupy space in the shell. The 

 nucleus shrinks by an amount equal to the combined igneous 

 intrusions and extrusions. I am inclined to believe that this 

 cause for crustal deformation is of the same order of magnitude 

 as that due to secular cooling. 



The fact that periods of considerable orogenic movements 

 generally correspond with periods of great vulcanism is very 

 suggestive and supports the conclusion as to the importance of 

 the above transfers of igneous material, in explaining crustal 

 corrugations. As a single illustration of this principle of cor- 

 respondence may be mentioned the fact that the great Tertiary 

 mountain-making period in which the Sierra Nevada range was 

 last uplifted, in which the Coast Ranges and St. Elias Alps were 

 formed, in which the Alps themselves were produced, and in 

 which other mountain ranges were formed, is contemporaneous 

 with the great Tertiary period of vulcanism. 



By the foregoing I do not mean to imply that vulcanism is 

 the initial cause of the orogenic movements. The initial causes 

 are those assigned for earth contraction. The transfers of 

 material followed as a result of the action of the initial causes, 

 and thus is in a measure an effect, but also where the transfer 



